1. Field of the Invention
This invention relates to the disposal of waste materials and, more particularly, to the permanent disposal of nuclear and toxic materials by depositing such materials in a subtending tectonic plate adjacent or as near as possible to a subduction zone.
2. Description of the Prior Art
The disposal of radioactive wastes from nuclear reactors and other atomic energy activities and of toxic byproducts caused by manufacturing and medical and biologic activities is an area of widespread concern. The long half-life of radioactive waste products and chemical compounds in which radioactivity is found presents a formidable obstacle to storage which will be inherently safe over the years. This is more clearly understood when it is realized that roughly 2.23 cubic meters of solid radioactive nuclear waste are produced annually by a conventional 1000 MW reactor. It is estimated that in the United States, the quantity of high-level radioactive waste generated by reactors to the present time would cover a football field to a height of three feet. Highly toxic Plutonium 239, which is included with this waste, has a half-life of approximately 25,000 years. Ten half-lives are required to reduce this radioactivity by a factor of one-thousand (1,000) which is generally considered to be the required safety level for exposure in the atmosphere. Thus, Plutonium 239 wastes should be isolated for a period of at least 250,000 years. Such toxic material must therefore be disposed in a location where it is impossible for the waste to find its way back into the environment for at least 250,000 years and, preferably, much longer. In respect of chemical wastes such as PCB's, however, they may retain their toxicity indefinitely and, therefore, it is desirable to ensure they remain undisturbed until their eventual destruction.
Presently, nuclear wastes are initially removed from a reactor and are placed in large vats of water while a cooling process takes place. Thereafter, they must be stored. Various techniques of storage have been considered including geologic repositioning within the continental crust and the implantation of solidified high-level waste or spent nuclear fuel into stable clay type sediments in low circulation regions in the mid-ocean. In addition, the construction of boreholes having the capability to store such wastes in the tectonic plate adjacent a subduction zone is described in U.S. Pat. No. 5,4,178,109 to Krutenat.
Such techniques, however, suffer inherent disadvantages. Nuclear wastes disposed of in a geologic repository on the continental crust have the potential to be tampered with by individuals or countries. Such wastes may accidentally be unearthed in the future by various actions and thereby become exposed to the environment. Wastes in a geologic repository also have the potential for intermingling with and contaminating the water cycle. Earthquake activity is also a problem in that it may fracture the geologic repository and release waste back into the environment. Volcanic activity, an act of war or sabotage, or impact by a celestial body could produce the same result.
A lack of international consensus or agreement is a major obstacle to the implantation of high-level radioactive waste containers in clay type sediments in the low circulation regions of the mid-ocean. Waste implanted in ocean sediments would also be subjected to natural upheavals and to mechanical perturbation once they eventually migrated to a subduction zone, as all seabeds are so predestined, as a portion of the sediment would be scraped off along the abutting continental edge. Wastes could then migrate back to the biosphere because of this abrasive action. Even if the sediment and embedded waste were subducted, the waste could return to the environment because of andesitic volcanism adjacent the subduction zone. This is so because it is believed that at a depth of near one hundred (100) kilometers within the earths crust, heat and pressure cause water to be driven from the crystalline structure of the subducted sediments. The heat generated by this phase change combined with the temperature of the rock at that depth causes some of the sediment and overlaying rock to melt and to rise to the surface as volcanoes. Waste melted along with the sediment could thereby return to the biosphere dissolved in the molten rock creating an undesirable environmental condition.
In the aforementioned U.S. Pat. No. 4,178,109, there is proposed a technique of disposing of wastes in boreholes at the edge of a subduction zone. While this is an improvement in the location of waste repositories, many problems remain inherent in this solution. Boring a single hole into the seabed from a platform on the surface of the ocean is a difficult and painstaking undertaking and hundreds of such boreholes would be required to accommodate world backlogs of high-level nuclear wastes because of the inherent size limitation caused by drilling. After construction of the borehole in the seabed, it would be difficult to relocate the hole and to deposit the waste into the hole. Such depositing would, apparently, require manipulation of the waste by apparatus located on the sea floor to fill the hole. This could not only be hazardous but an accident while filling the hole could scatter radioactive debris over the seabed. The waste, probably, would also inherently be required to be unshielded when deposited, again because of the diameter of the borehole which would prohibit protective sheathing from being inserted with the waste.
Likewise, the problem of scouring mechanical action as the subtending oceanic crust scraped against the non-descending crust would create problems since waste implanted in boreholes in the oceanic crust any distance from the originating ridge would likely be necessarily implanted in the sedimentary layer. This sedimentary layer is, on average, three (3) to four (4) kilometers thick at the subduction zone.
In respect of toxic wastes such as chemical, medical and biological wastes, typical previous disposal techniques include incineration and burial or dumping of such wastes in the sea. These are also disadvantageous.
Incineration of toxic wastes requires the process to be conducted within exacting tolerances. Otherwise, the potential for generating other poisons, which may be even more hazardous than those originally intended for disposal, exists. Even when carried out under ideal conditions, incineration is inherently atmospheric polluting.
Burying toxic wastes and low-level radioactive wastes has also proven disadvantageous. There have been instances where buried wastes have percolated through the overburden meant to isolate it, thereby contaminating the overlaying property such as the Love Canal, in upstate New York, U.S.A. Buried wastes have frequently been inundated by or have themselves seeped into subterranean aquifers thereby fouling the fresh water supply.
Medical and other wastes thought to have been eliminated when dumped at sea frequently have washed ashore and have received widespread publicity in doing so.
The earth's crust is formed of large solid tectonic plates. These large tectonic plates are formed at ocean ridges and slowly migrate until they reach "subduction" zones at which location they re-enter the earth at an average rate of six (6) cm per year.
An objective of the present invention is to place waste material in repositories radiating outwardly from an access tunnel bored into the basaltic layer of the oceanic crust beneath sediments overlaying the basaltic layer at or as near as possible the edge of a subduction zone. The access tunnel would originate from land on the nondescending side of a subduction zone, from the surface of the subducting plate itself, from a man-made or naturally formed island situated over a tectonic plate that is moving towards a subduction zone or from a caisson situated over the subtending tectonic plate. Each repository filled with waste would be sealed from the access and, accordingly, the biosphere, by a plug. The crustal downwards movement of the tectonic plate would carry the waste into the interior of the earth. Many millions of years would be required for the waste to circulate through the earth's mantle before it could reemerge in a diluted, chemically and physically altered form at an oceanic ridge.
There are several areas located throughout the world that are favorable locales for the tunnel and repository process described herein. In Canada, the Brooks Peninsula on Vancouver Island in the Province of British Columbia, Canada and the Scott Islands north of Vancouver Island are located on the non-descendlng, North American Plate side of the Cascadia subduction zone. They are located near enough to the subducting Explorer Plate to make accessing the subducting plate by a tunnel with an origin on the North American side of the subduction zone possible. The subduction zone is also shallow enough, in the range of 1 mile, opposite these sites to permit successful tunneling beneath the Pacific Ocean.
In the United States, Cape Mendecino north of San Francisco in the State of California is similarly situated but located a greater distance from the subducting zone. A tunnel from Cape Mendecino into the subducting Gorda Plate would likely be of similar dimensions to the one recently completed in Japan to link the islands of Honshu and Hokkaido and to the tunnel between France and Great Britain presently under construction. Accordingly, the feasibility of constructing such a tunnel has been demonstrated.
In New Zealand, the subduction of the Pacific Plate beneath the Indo-Australian Plate takes place partially on the North Island. To implement the process according to the invention in New Zealand, a tunnel would only have to be pushed far enough into the Pacific side of the North Island that waste deposited in repositories radiating from it would not be encountered accidentally by mineral or petroleum prospectors in the future.
The Hawaiian and Mariana Islands are situated above tectonic plates moving towards subduction zones. Tunnels from these or similar islands could access repositories in an oceanic plate which would be subducted at some predictable time in the future.
It may also be feasible to construct a man-made island as near as possible to the subduction zone so that repositories could be accessed via a tunnel, originating on the island, constructed into the subducting plate. Such an island, for example, has been constructed in the Beaufort Sea and used as a base for drilling exploratory oil and gas wells.
A caisson could also be used to access a shallow tectonic plate near a subduction zone so that repositories could be radiated from an accessing tunnel constructed from the caisson which would act similar to a man-made island.
Once accessed, the subducting plate could yield as many waste repositories as necessary to eliminate current waste backlogs as well as future requirements. Each repository, once filled, would be sealed from the tunnel access and, accordingly, the biosphere by a plug.